Phylum Gastrotricha

Chapter 12

Phylum Gastrotricha Tobias Kånneby Department of Zoology, Swedish Museum of Natural History, Stockholm, Sweden

Rick Hochberg Department of Biological Sciences, University of Massachusetts, Lowell, MA, USA

Chapter Outline Introduction211 General Systematics 211 Phylogenetic Relationships 212 Distribution and Diversity 214 General Biology 214 External Anatomy 214 Internal Anatomy and Physiology 216 Reproduction217

INTRODUCTION Gastrotricha is a small group of acoelomate invertebrates common to most aquatic habitats. The group constitutes almost 800 species worldwide, of which close to 350 nominal species have been found in freshwater, where they are a common component of the benthos, the periphyton, and to a lesser extent the interstitial. Marine taxa are almost exclusively interstitial. Freshwater gastrotrichs are among the top five most common taxa encountered, with densities in the range of 100,000 to 1,000,000 m−2. However, due to their minute size (60–800 μm), and transparent colorless bodies, they are usually overlooked. There is very little known about how gastrotrich distribution and abundance are controlled in nature. They are, however, hypothesized to be an important link between the microbial loop and larger invertebrate predators (Balsamo and Todaro, 2002). Although not apomorphies, most freshwater gastrotrichs can be recognized by the following combination of characteristics: (1) ventral ciliation, usually distributed in two longitudinal bands; (2) a terminal or subterminal mouth connected to a muscular sucking pharynx; and (3) a posterior fork-like structure (furca) that bears distal adhesive tubules. In all, three families—Chaetonotidae, Dasydytidae, and Neogosseidae—spanning 12 genera and approximately 70 species, have been reported from the Nearctic region

General Ecology and Behavior 218 Habitat Selection 218 Physiological Constraints 218 Feeding Behavior 220 Predators and Parasites 220 Collecting, Culturing, and Specimen Preservation 220 References221

(Schwank, 1990; Balsamo et al., 2008). It is obvious that the Nearctic fauna have received little study and that these numbers underestimate the real diversity of the fauna when compared to those reported from well-investigated countries in Europe (Poland, 98 spp.; Italy, 92 spp.; Russia 91 spp.; Romania, 90 spp.; Germany, 90 spp.) (Balsamo et al., 2008). Important general references on gastrotrichs include Remane (1935–36), Hyman (1951), Voigt (1958), d’Hondt (1971), Hummon (1982), Ruppert (1988, 1991), Schwank (1990), and Balsamo and Todaro (2002).

General Systematics Gastrotricha is currently considered a phylum in the subclade Platyzoa within the superclade Lophotrochozoa (Todaro et al., 2006; Hejnol et al., 2009). Current classification divides the phylum into two orders, Chaetonotida and Macrodasyida. Chaetonotida is found in both freshwater and marine habitats, while Macrodasyida is, with very few exceptions, entirely marine. Macrodasyidans are distinguished from species of Chaetonotida by the presence of pharyngeal pores and generally by a strap-shaped body with more than two pairs of adhesive tubules that are not limited to the posterior end (Figure 12.1). Chaetonotida are usually tenpin- or sack-shaped and possess one or rarely two pairs of adhesive tubules limited to

Thorp and Covich’s Freshwater Invertebrates. http://dx.doi.org/10.1016/B978-0-12-385026-3.00012-7 Copyright © 2015 Elsevier Inc. All rights reserved.

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FIGURE 12.2  Schematic illustration of a typical chaetonotidan gastrotrich showing some of the major anatomical features. (AG, adhesive glands; AT, adhesive tubes; EG, egg; G, gut; M, mouth; PH, pharynx; PN, protonephridium; SC, sensory cilia; SP, sperm; X, x-organ.) Modified from various sources. FIGURE 12.1  Freshwater macrodasyidan gastrotrichs. (a) Marinellina flagellata. (b) Redudasys fornerise. (AT, adhesive tube; EG, egg; PP, pharyngeal pore.) Based on Ruttner-Kolisko, 1955; Tirjaková, 1998; and Todaro et al., 2012.

the posterior end (Figure 12.2). The entirely marine family Neodasyidae shares several morphological characters with Macrodasyida but is currently placed within Chaetonotida. Recent molecular phylogenetic studies (Todaro et al., 2006) have, however, questioned this classification. To date, only two species of freshwater gastrotrichs are placed within Macrodasyida. Ruttner-Kolisko (1955) described an aberrant gastrotrich, Marinellina flagellata, from an Austrian stream (Figure 12.1(a)). Unfortunately, the species was described from only two immature specimens and the taxon is currently considered incertae sedis. Kisielewski (1987) described the species Redudasys fornerise Kisielewski, 1991, an unquestionable macrodasyidan, from the psammon of a Brazilian reservoir (Figure 12.1(b)). This taxon was also considered incertae sedis until Todaro et al. (2012) confirmed the position of R. fornerise using molecular data, and subsequently placed the family Redudasyidae within Macrodasyida. Several macrodasyidans are currently known from brackish water, and it is very plausible that additional freshwater Macrodasyida will be found when the appropriate habitats (psammon, hyporheic zone, etc.) are explored. Distribution, biology, evolutionary relationships, and the invasion of freshwater habitats of such species will be of great interest.

Chaetonotida have successfully colonized freshwater environments, and five families represented by 23 genera are currently known from freshwater habitats (Figure 12.3). Several genera can be found in both freshwater and marine habitats (e.g., Aspidiophorus, Chaetonotus, Heterolepidoderma, and Ichthydium). The most speciose and evolutionarily successful family, Chaetonotidae, has recently been shown to be a non-monophyletic taxon since at least the semi-planktonic families Dasydytidae and Neogosseidae are nested within the group (Hochberg and Litvaitis, 2000; Kånneby et al., 2012, 2013).

Phylogenetic Relationships Gastrotricha was long considered a group within the Aschelminthes or Nemathelminthes, a now obsolete group of triploblastic, vermiform animals comprising Rotatoria, Nematoda (= Nemata), Nematomorpha, Acanthocephala, Priapulida, and Kinorhyncha (Hyman, 1951). The phylogenetic position of Gastrotricha is still somewhat unclear, and certain authors regard it as a sister group of the Ecdysozoa (Schmidt-Rhaesa et al., 1998) or a clade within Cycloneuralia (Nielsen, 2001). Recent molecular phylogenetic and phylogenomic studies have rejected a relationship to Ecdysozoa and instead placed the group within Platyzoa in close alliance to Micrognathozoa, Cycliophora, or Rotifera (Hejnol et al., 2009).

Chapter | 12  Phylum Gastrotricha

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M

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U

G

O

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Q

R

S

X

Y

Z

FIGURE 12.3  Freshwater chaetonotidan genera representing Chaetonotidae (a–k), Dasydytidae (l–r), Dichaeturidae (s), Neogosseidae (t–u), and Proichthydidae (v–w). (a) Arenotus, (b) Aspidiophorus, (c) Chaetonotus, (d) Fluxiderma, (e) Heterolepidoderma, (f) Ichthydium, (g) Lepidochaetus, (h) Lepidodermella, (i) Rhomballichthys, (j) Polymerurus, (k) Undula, (l) Anacanthoderma, (m) Chitonodytes, (n) Dasydytes, (o) Haltidytes, (p) Ornamentula, (q) Setopus, (r) Stylochaeta, (s) Dichaetura, (t) Kijanebalola, (u) Neogossea, (v) Proichthydioides, (w) Proichthydium. Adapted and modified from Strayer et al. 2009.

Gastrotricha is considered monophyletic based on the following apomorphies: a multilayered cuticle, locomotory and sensory cilia covered by epicuticle, duo-gland adhesive cells that form cuticle-covered tubes, and peculiar helicoidal muscles surrounding the alimentary canal. Interrelationships are difficult to discern, but three major clades of gastrotrichs are widely recognized; the order Macrodasyida and the order Chaetonotida, the latter of which contains two suborders, Multitubulatina and Paucitubulatina. Hochberg and Litvaitis (2000) hypothesized monophyly for all four taxa using morphological

cladistics, reflecting their current classification. A later morphological analysis by Kieneke et al. (2008) questioned some of these findings using a larger dataset and more species, and molecular studies have suggested Macrodasyida as a potentially paraphyletic taxon containing a monophylyetic Chaetonotida (Todaro et al., 2006). Within Chaetonotida, taxonomy is unstable, and current classification is to a great extent based on the distribution and shape of cuticular structures such as scales and spines. These characteristics are variable and differ even within the same morphological species (Amato and Weiss, 1982). There is

214

also a growing concern that characteristics intermediate between traditionally defined genera within Chaetonotidae may blur distinctions between genera such as Chaetonotus, Heterolepidoderma, Ichthydium, and Lepidodermella. In response, Schwank (1990) and Kisielewski (1997) erected new genera and subgenera and redefined existing genera based on morphology. Recent molecular studies have tested the monophyly of some of these groupings, from the family to the species level. Kånneby et al. (2013) demonstrated that most of the long-established genera within Paucitubulatina are non-monophyletic groups and that cuticular structures do not reflect phylogenetic relationships. Use of cuticular structures is, however, inevitable when diagnosing, describing, and identifying species. The same study also found that marine and freshwater species that currently belong to the same genus do not form monophyletic groups, which points to the convergent evolution of cuticular structures. This finding demonstrates the need for a reclassification of the Paucitubulatina based on other characters. Further difficulty comes with the recognition that cryptic speciation may be a common phenomenon among freshwater Chaetonotida. Highlighing this problem is perhaps the most well-studied freshwater species, Lepidodermella squamata (Dujardin, 1841), which is a common and apparently cosmopolitan species that is also sold commercially. Kånneby et al. (2012) revealed that L. squamata likely consists of at least two cryptic species, and perhaps more, when additional specimens are examined. Classification within Macrodasyida is based on a larger suite of characteristics (e.g., cuticular, glandular, and reproductive) and is generally considered more reliable than that of Chaetonotida; families are better defined and follow the cladistic classifications hypothesized by various researchers (Hochberg and Litvaitis, 2000; Kieneke et al., 2008).

Distribution and Diversity Gastrotricha is a cosmopolitan group that has been reported from all major continents. Based on the history of gastrotrich research, most freshwater species have been reported from Europe (e.g., Schwank, 1990). Some countries with substantial land area, such as Brazil, have received significant attention (Kiselewksi, 1991), while others such as Argentina, Australia, India, and most of Africa have received only cursory studies (reviewed in Balsamo et al., 2008). The picture is somewhat different when one limits the assessment to freshwater gastrotrichs of the Nearctic, especially considering the biogeographic diversity of this large area. There are numerous ecological and taxonomic studies from individual lakes across the USA, mostly concentrated in the Midwest (e.g., Brunson, 1949, 1950; Robbins, 1973). Smaller waters bodies have also been well studied with regards to ecological factors, such as Strayer’s (1985)

SECTION | III  Protozoa to Tardigrada

studies in New Hampshire and the study by Weiss (2001) of reproductive biology in New Jersey. To date, there are several reports of species in Canada (Schwank, 1990). Except for these records, the rest of the gastrotrich fauna of Nearctic freshwaters remain poorly known. By far the most diverse groups of freshwater Gastrotricha in the Nearctic are Chaetonotus and Ichthydium, with almost 30 and 9 recorded species, respectively. This number is still low considering the 210 or so nominal species known collectively for these genera. Several species of the semiplanktonic families Dasydytidae and Neogosseidae have also been described and recorded from the region. Certain species are uncommon and have so far not been recorded from any other region than the Nearctic or the country from which they were originally described.

GENERAL BIOLOGY The following accounts of external anatomy, internal anatomy, and physiology are based on Remane (1935–36), Hyman (1951), Hummon (1984), Schwank (1990), and Ruppert (1991), which should be consulted for greater detail.

External Anatomy Gastrotrichs are very small and colorless animals. As a rule of thumb, Macrodasyida are generally larger and bulkier than Chaetonotida. The macrodasyidan Megadasys pacificus Schmidt, 1974 can attain lengths of up to 3.5 mm, while the largest freshwater chaetonotidan can reach only 0.77 mm. The smallest gastrotrich is Chaetonotus spinulosus Stokes, 1887, which reaches a total body length of 60 μm. However, most freshwater species are generally larger, 100–300 μm in total body length. Their small size and transparent bodies make most chaetonotidans difficult to discover in benthic samples. Most freshwater Chaetonotida are tenpin-, bottle-, or sack-shaped (Figure 12.3). The head consists of one to five lobes that are usually covered by cuticular plates known as the cephalion (dorsal), epipleura (lateral), and hypopleura (lateral). Sensory cephalic tufts originate between these plates. Certain species also have (presumably) photosensitve ocellar granules, usually located between the borders of the epi- and hypopleura (Figure 12.4(b)). The head is often separated from the trunk region by a neck constriction. In the trunk region, the lateral body sides may be parallel as in thin or elongated species, or convex as in stouter species. A pair of sensory bristles is usually present in the neck region and the posterior trunk region. The posterior pair is often anchored by special three-lobed scales, which differ from other scales of the same area, or seldom by a papilla. The trunk gradually tapers into the furca. The furca is a fork-like structure that carries distal adhesive tubules. In most species

Chapter | 12  Phylum Gastrotricha

215

D

E

H

I

J

L

F

G

K

M

N

FIGURE 12.4  Freshwater chaetonotidan gastrotrichs. (a) Chaetonotus hystrix Metschnikoff, 1865, dorsal view. (b) Heterolepidoderma macrops Kisielewski, 1981, habitus. (c) Ichthydium skandicum Kånneby et al., 2009, habitus. (d) Ichthydium squamigerum Balsamo and Fregni, 1995, habitus. (e) Heterolepidoderma ocellatum (Metschnikoff, 1865), posterior end showing packets of sperm (arrow) and X-organ (arrowhead). (f) Lepidochaetus zelinkai (Grünspan, 1908), lateral view. (g) Lepidodermella squamata (Dujardin, 1841), habitus of juvenile specimen. (h) Polymerurus nodicaudus (Voigt, 1901), view of anterior body region showing an ingested diatom. (i) Haltidytes festinans (Voigt, 1909), habitus. (j) Ornamentula paraënsis Kisielewski, 1991, dorsal view. (k) Neogossea antennigera (Gosse, 1851) anterior portion showing head tentacles. Photos: T. Kånneby.

216

only two adhesive tubules are present; but in some rare groups (Dichaeturidae, Marinellina, and Redudasys), the furca may be doubly branched with four adhesive tubules. The semi-planktonic Dasydytidae and Neogosseidae lack the furca and adhesive tubules altogether (Figure 12.3). The ventral side of the body is more or less flat and bears locomotory cilia, which are distributed in longitudinal bands or tufts. The dorsal side of the body is usually arched. The whole body, including locomotory and sensory cilia, is covered by cuticle (only the exocuticle covers the cilia, though), which can be sculpted into a wide array of cuticular structures. Cuticular structures are extremely important in species identification and consist of smooth, keeled, pedunculated, ornamented, or spined scales of different shapes and sizes (Figures 12.3 and 12.4). Spines are the hallmark for species of Chaetonotus but are also present in other groups (e.g., Dasydytidae and other genera of Chaetonotida). Scales may be rigid or thin and bendable, and/or dentated. Species of select genera do not possess any apparent cuticular structures (e.g., Arenotus, Ichthydium, Redudasys, and Marinellina), but high-resolution and electron microscopy have shown that at least some species of Ichthydium possess minute cuticular structures.

Internal Anatomy and Physiology The bilayered cuticle is secreted by the epidermis, and lines both the external anatomy and the lumen of the myoepithelial pharynx. The epidermis is cellular in most Macrodasyida and mostly syncytial in Chaetonotida. Certain epidermal cells of the ventral side are ciliated and used for locomotion. In some taxa (e.g., Dactylopodolidae and Neodasyidae) these cells are monociliated, but in most species they are multiciliated. The epidermis may be further differentiated into general epithelial cells, specialized gland cells, and some sensory cells (ciliated). In Chaetonotida, the most distinct gland cells are the posterior adhesive tubes, which consist of two cells—a single adhesive cell and single release cell—together covered by the bilayered cuticle that forms a tube. The muscular system includes a series of individual muscle bands that are oriented in four configurations: circular, longitudinal, dorsoventral, and helicoidal. In Chaetonotida, circular muscles are generally only present around the pharynx and probably function to oppose pharyngeal dilations during feeding. Longitudinal bands immediately outside the circular muscles extend from anterior to posterior and function in general bending movements. Some bands are branched in the trunk region and may function to hold developing parthenogenetic eggs as they grow. Helcoidal muscles, which form a double helix-like pattern down the length of the body, wrap around the circular and longitudinal muscles of the pharynx and may enclose some longitudinal bands in the trunk region. Dorsoventral

SECTION | III  Protozoa to Tardigrada

muscles, when present, are restricted to the trunk region. Some semiplanktonic species (Dasydytidae) can abduct their spines with specialized oblique and segmented lateral muscles, which presumably allows for some limited saltation. Reproductive organs of most macrodasyidans are ­muscular, while those of chaetonotidans lack muscles. The alimentary canal is complete and relatively unspecialized. The round to oval mouth is situated terminally or subterminally in the anterior end (Figure 12.2). The mouth ring may contain ridges, denticles, or other sharp structures that facilitate prey capture. In Chaetonotida, a short buccal capsule containing longitudinal ridges and sharp teeth may be present between the mouth and the pharynx. The pharynx is an elongated myoepithelial tube approximately 1/6 to 1/3 of the total body length and similar in construction to the nematode pharynx. The pharynx lumen is Y-shaped in Chaetonotida and inverted Y-shaped in Macrodasyida. Members of Macrodasyida have pharyngeal pores (except Lepidodasyidae) situated in the posterior part of the pharynx. In Chaetonotida, bulbous enlargements may be present in the anterior and/or posterior end of the pharynx. The pharynx is connected to a straight intestine via a pharyngeal-intestinal valve that probably functions to prevent backflow of food. The intestine is unbranched and ends in a subterminal ventral anus that is usually covered by the ventral terminal scales in Chaetonotida. The gastrotrich nervous system is divided into central (CNS), stomatogastric (SN), and peripheral (PN) components. The CNS consists of a circumpharyngeal cerebral ganglion and paired or multiple nerve cords; the SN innervates the pharynx and intestine; and the PN innervates sensory structures of the body. The cerebral ganglion is relatively large and forms a lobe on each side of the pharynx. These lobes are connected dorsally and by a small commissure ventrally. Each lobe gives rise to one or more lateral cords that innervate the head anteriorly and extend to the posterior part of the body. Commissures connect the cords in the trunk region. Sensory structures are generally simple and often tactile, chemoreceptive, and/or possibly rheoreceptive. They occur as spines, bristles, cilia, and tentacles and are concentrated on the head region but may present all over the body. In chaetonotidans, sensory bristles are present in the neck region and posterior trunk region. Ciliary tufts on the head act as tactile receptors but may also be used for locomotion (Figure 12.2). Certain chaetonotidans have ciliated depressions or furrows on the head that may be chemosensory in function (Schwank, 1990); macrodasyidan gastrotrichs often have more elaborate structures (e.g., tentacles, palps, pits) that play a chemoreceptive role. Several gastrotrichs have photoreceptors with pigmented granules that presumably act as filters— e.g., Heterolepidoderma ocellatum (Metschnikoff, 1865), Aspidiophorus oculifer Kisielewski, 1981, and Chaetonotus oculifer Kisielewski, 1981—but their structure has not

Chapter | 12  Phylum Gastrotricha

been examined. In the marine macrodasyidan, Dactylopodola baltica (Remane, 1926), at least two types of photoreceptors are present—pigmented ocelli and non-pigmented ocelli-like structures—and may play different roles in phototactic behavior (Liesenjohann et al., 2006). In fact, the common freshwater chaetonotidan, Lepidodermella squamata, shows a preference for blue light despite the lack of any pigmented ocelli (Balsamo, 1980). Excretory organs consist of protonephridia (Figure 12.2). In Paucitubulatina, paired protonephridia each consist of one flame cell (terminal organ) and one elongate canal cell that may be highly convoluted. Neodasys and species of Macrodasyida possess several pairs of protonephridia, and the number of flame cells varies and may increase with age. There are no special respiratory or circulatory organs in gastrotrichs. They are small animals and respiration takes place by diffusion through the body wall.

Reproduction Until rather recently, populations of freshwater gastrotrichs were thought to consist solely of parthenogenetic females. However, more detailed and recent studies have shown that freshwater gastrotrichs have a more complex life-cycle consisting of both parthenogenetic and hermaphroditic phases (Weiss, 2001; Figure 12.5). Gastrotrichs undergo direct development, and most freshwater Chaetonotida produce four to five eggs that are laid sequentially over a 4–5 day period. Ova mature in an anterior direction and are not surrounded by an oviduct; the most anterior egg matures the fastest and generally becomes positioned dorsal to the intestine. While the details of oviposition are not

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well known, it is assumed that it generally occurs through rupture of the body wall. Chaetonotidan eggs are some of the largest in the animal kingdom relative to adult body size and can occupy more than 75% of the trunk region in some specimens (Figure 12.4(a and b)). Newly hatched neonates are relatively large, approximately 2/3 of the length of adults, and already contain developing parthenogenetic eggs. Under favorable conditions, eggs may develop rapidly; at 20 °C, the first egg may be laid just a day after the mother herself hatches. Parthenogenesis appears to be strictly apomictic. Embryogenesis appears to follow a modified radial cleavage pattern that differs between Chaetonotida and Macrodasyida. In general, gastrotrich eggs are oval and ornamented, sometimes only on one side. Eggs are usually deposited in the surface film or on the benthos, where the ornamentation may help the egg to adhere to the substratum (Hyman, 1951). There are two types of parthenogenetic eggs in Chaetonotida. The tachyblastic egg develops first and hatches quickly after oviposition. Occasionally, the final egg is an opsiblastic egg or dormant egg, which is a little larger and thicker-shelled than the tachyblastic egg and very resistant to freezing and drying. Factors that induce the production of opsiblastic eggs are not well known, but such eggs are often produced by animals in crowded cultures. After the initial parthenogenetic eggs are laid, the parent gradually develops into a simultaneous hermaphrodite (Hummon and Hummon, 1983). During this post-parthenogenetic phase, sperm and meiotic eggs are present in the body (Figure 12.5). An accessory reproductive organ called the x-organ, which is horseshoe shaped and of unknown function, develops in the posterior end during gametogenesis. To date, no studies have confirmed any form of mixis, but Levy and Weiss (1980) observed a third kind of egg, the

FIGURE 12.5  Schematic diagram of the proposed generalized life-cycle for freshwater chaetonotidan gastrotrichs, based predominantly on studies of Lepidodermella squamata. Dashed lines show hypothetical events that have not yet been demonstrated. Adapted from Strayer et al., 2009.

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“plaque bearing egg,” in cultures of Lepidodermella squamata and suggested it to be the product of sexual reproduction. Sperm are few in number, 32 or 64 per animal, and nonmotile; it is probable that fertilization is internal and may be automictic. The gastrotrich life-cycle is unique among invertebrates and offers considerable ecological flexibility to gastrotrich populations (Figure 12.5). Under favorable laboratory conditions, the initial parthenogenetic phase allows for quick population growth. The dormant eggs allow the population to survive unfavorable environmental conditions and may also act as means of dispersal to new habitats. The subsequent hermaphroditic phase ensures genetic recombination and probably occurs in populations in which mortality rates are low enough to allow some individuals to reach the age required for sexual development.

GENERAL ECOLOGY AND BEHAVIOR Habitat Selection Gastrotrichs are widely distributed in most freshwater habitats and are common in surface sediments and among anchored and floating vegetation. Certain species of the Neogosseidae and Dasydytidae are good swimmers and also float well. These groups have occasionally been reported from the plankton of shallow weedy lakes (Kisielewski, 1991). There are, however, no truly planktonic gastrotrichs to compare with the cladoceran crustaceans or ploimate rotifers. Gastrotrichs have been reported from a wide range of freshwater and semi-terrestrial habitats. Most species are abundant in lakes, ponds, and wetlands (Tables 12.1 and 12.2). Highly organic sediments usually have a high gastrotrich density (Strayer, 1985). Kisielewski (1986) found that gastrotrich density and species richness are positively correlated with the productivity of the habitat. Many groups of freshwater gastrotrichs have an intercontinental or cosmopolitan distribution. Some taxa have more restricted distributions, such as the chaetonotidans Arenotus, Undula, and the dasydytid Ornamentula. Dichaeturidae is so far only known from Europe, while the rare Proichthydium and Proichthydioides have only been found at their type localities in Argentina and Japan, respectively. The geographic distribution of individual species is less well known, but several species such as Lepidodermella squamata, Chaetonotus hystrix Metschnikoff, 1865, Chaetonotus maximus Ehrenberg, 1838, and Ichthydium podura Müller, 1773 are widely distributed and have been reported from most continents. There are other reported species with apparently broad geographic ranges, and in many cases they are known from more than one continent. At the species level, 30% of the European fauna and 50% of the South American freshwater gastrotrich fauna appear cosmopolitan or intercontinental (Balsamo et al., 2008). However, when

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compared morphologically, many species found on two or more continents are often somewhat different (Kisielewski, 1991); and with the almost unknown extent of cryptic speciation within freshwater Chaetonotida, care should be taken when describing potential cosmopolitans (Kånneby et al., 2012). Still, freshwater gastrotrichs can probably disperse over wide geographical areas, especially as dormant stages (resting eggs) or possibly in the feathers or fur of migratory birds and mammals (Schwank, 1990). Generally, gastrotrichs live close to the sediment surface in lakes (Figure 12.6) and are also common in the interstitial of sand and gravel bars of unpolluted streams where they can penetrate deep into the sediments. Sphagnum bogs and small still waters with Lemna are usually rich and diverse in gastrotrich species composition, and one or two species are dominant while several other taxa are subordinate. Certain species have adapted to semi-terrestrial habitats and have been found in moist leaf litter, mosses, and epiphytic Bromeliaceae (Kisielewski, 1991). Apparently, freshwater gastrotrichs are scarce in groundwaters other than the hyporheic zone. Chaetonotus typically dominates the gastrotrich fauna everywhere in freshwater, both in terms of number of species and their overall abundance (Table 12.1), and is the only freshwater genus known from Antarctica other than Lepidodermella (Sudzuki and Shimoizumi, 1967). Other genera of Chaetonotidae, including Ichthydium and Polymerurus, are also common in all kinds of freshwaters but not as abundant as Chaetonotus. Dasydytidae and Neogosseidae are less widespread than the Chaetonotidae and are more diverse in the tropics than in temperate waters (Kisielewski, 1991; Table 12.1); they may be very common in weedy productive waters.

Physiological Constraints Gastrotrichs are among the few animals commonly found in anaerobic environments and are abundant even after months of extended anoxia (Strayer, 1985). The physiological basis of anaerobiosis in freshwater gastrotrichs has not yet been studied. It seems plausible that some gastrotrichs possess a sulfide detoxification mechanism similar to that demonstrated for marine gastrotrichs and freshwater nematodes. Also, little is known about the factors that control the distribution of individual species of gastrotrichs in freshwater. Based largely on potentially analogous marine studies, such as d’Hondt (1971), one might assume that the factors of primary importance include granulometry, stability, packing, organic content of the sediment, amount of dissolved oxygen, and density and composition of communities of microbes and predators. Culture work also suggests that the inorganic chemistry of the water as well as anthropogenic contaminants can exert a

Chapter | 12  Phylum Gastrotricha

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TABLE 12.1  Number of Species of Gastrotrichs Found in Some Freshwater Habitats Habitat

Total

Chaetonotus

European lakesa

18

12

Mirror Lake, NH

20–32

8–20

Ponds, Polandb

21

Peat bogs, Polandc Polandd

Other Chaetonotidae

Dasydytidae

Neogosseidae

Source

5

<1

0

Balsamo (1981), ­Bertolani and Balsamo (1989), Preobrajenskaja (1926)

12

0

0

Strayer (1985)

10

7

3

<1

Kisielewski (1986), Nesteruk (1986)

27

16

9

2

0

Kisielewski (1981)

24

12

8

4

0

Kisielewska (1982)

Phragmites mats, Romania

28

16

7

5

0

Rudescu (1968)

Ponds, Brazil

38

14

12

10

2

Kisielewski (1991)

Rivers, Brazil

22

5

12

4

1

Kisielewski (1991)

Bog pools,

aMean

of four lakes. of seven ponds. cMean of four bogs. dMean of two pools. bMean

TABLE 12.2  Density and Biomass of Gastrotrichs in Some Freshwater Habitats Density (No./m2)

Biomass (mg DM/m2)

1,160,000a

23a,b

Nesteruk (1996)

Lake Brzeziczno, 920,000a Poland

30a,b

Nesteruk (1996)

Lake Piaseczno, Poland

910,000c

23b,c

Nesteruk (1996)

Mirror Lake, NH

130,000d

1d

Strayer (1985)

Lake Suviana, Italy

57,000e

–

Madoni (1989)

Lake Erie, OH

50,000f

–

Evans (1982)

Three small ponds, Poland

1,600,000– 2,600,000

25–78b

Nesteruk (1996)

–

Hummon (1987)

Site Lake Biczce, Poland

Mississippi River, 130,000– MN 230,000g aLittoral

zone. from wet mass by multiplying by 0.15. cMean of three stations. dLakewide mean. eMean of two deepwater stations. fBeaches. gSand bars. bConverted

Source

FIGURE 12.6  Vertical distribution of gastrotrichs within the sediments of Lake Brzeziczno, Poland (dashed line) and Mirror Lake, New Hampshire, USA (solid line) as a function of depth from the sediment surface. Based on data from Nesteruk, 1996 and Strayer, 1985. Adapted from Strayer et al., 2009.

SECTION | III  Protozoa to Tardigrada

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strong influence on gastrotrich populations (Hummon and ­Hummon, 1979). In laboratory cultures, gastrotrichs are capable of enormous population growth rates (10–50% per day), but there is no direct information on what factors regulate populations in nature. If potential growth rates are anywhere near this high in nature, there must be an equally high counterbalancing mortality, perhaps from predation. The few quantitative studies of seasonal dynamics of freshwater gastrotrich populations have shown that densities are usually lower during the winter (Strayer, 1985; Figure 12.7). Seasonal changes of water temperature, prolonged freezing, food supply, and predation pressures are obvious possibilities for seasonal dynamics.

Feeding Behavior Gastrotrichs feed on bacteria, algae, protozoans, detritus, and small organic particles (Figure 12.4(h)) and they are hypothesized to be an important link between the microbial loop and larger invertebrates (Schwank, 1990; Balsamo and Todaro, 2002). Food particles are sucked in through the mouth by powerful contractions of the pharyngeal myoepithelium. In species with cuticular elaborations of the mouth and buccal capsule, food may be partly macerated prior to entering the pharynx lumen. Extracellular digestion is likely restricted to the intestine. Bacteria are probably of primary importance in the gastrotrich diet, and Bennett (1979) showed that Lepidodermella squamata would not survive in laboratory cultures without bacteria. The same study demonstrated that L. squamata also consumes the green alga Chlorella, which may be of secondary importance to the otherwise bacterial diet. Freshwater gastrotrichs are probably capable of fine

discrimination by different bacterial cultures. Gray and Johnson (1970) showed that the marine macrodasyidan, Turbanella hyalina could choose among various strains of bacteria, apparently using some kind of tactile and/or chemical sense.

Predators and Parasites Predators of gastrotrichs include heliozoans, sarcodine amebae, cnidarians, and tanypodine midges, but there are presumably many other benthic predators that feed on gastrotrichs. Nothing is known about the quantitative importance of gastrotrichs as food items for various predators. There are no reports of parasites in freshwater gastrotrichs. However, microsporidians are known from marine macrodasyidans (Manylov, 1999); and it seems likely that once freshwater species are thoroughly examined, parasitic protozoa and fungi will be the likely culprits.

COLLECTING, CULTURING, AND SPECIMEN PRESERVATION Freshwater gastrotrichs are easily collected from different water bodies using a fine-mesh plankton net (20–30 μm). Sediments and vegetation can also be collected by hand. Individuals can be concentrated by squeezing the water of Sphagnum mosses into a jar. Interstitial gastrotrichs are collected by scooping up the top layer of the sandy/silty sediment. Extraction of gastrotrichs from a sample can be difficult. Some workers have handpicked and counted animals under the dissecting microscope, which is a tedious affair. Density

50

Density

20

25 10

J J A S O N D J F M A M

J-J

(number/cm3)

A-S

O-D

J-M

A-M

FIGURE 12.7  Seasonal trends in gastrotrich abundance. Left: density of two species of dasydytids in the surface sediments of a boggy pool (“Complex B”) in Poland: Setopus (●) and Dasydytes ornatus (Voigt, 1909) (Δ). From data in Kisielewska (1982). Right: density (number/cm2) of all gastrotrichs (●), an unidentified species (probably of Heterolepidoderma) (□) and Lepidodermella triloba (Brunson, 1950) (■) on the gyttja sediments of Mirror Lake, New Hampshire (Strayer, 1985). Plotted points are means ± s.e. (n = 16). Adapted from Strayer et al., 2009.

Chapter | 12  Phylum Gastrotricha

gradient centrifugation may be useful but has not yet been tested for freshwater gastrotrichs. Nesteruk (1987) found that a modified Baermann funnel was useful for extracting chaetonotidans (other than species of Dasydytidae) from pond sediments. Sieves should be used cautiously because gastrotrichs are often too small to be retained quantitatively on fine-mesh sieves. For interstitial species, quantative samples can be taken with a small diameter (2–5 cm) core. For epiphytic species, quantative samples can probably be obtained by modifying sampling methods developed for macroinvertebrates, i.e., use an extra-fine mesh net (<20 μm), a small sample volume, or subsampling. Gastrotrichs should be processed alive for taxonomic work. If, however, the samples need to be preserved immediately, you should anesthetize animals with a 1–2% magnesium chloride solution and subsequently fix them in 10% formalin with Rose Bengal. In taxonomic work where only few specimens are needed, samples with a high organic content (e.g., mosses, plants, organic sediment) can be kept in aerated aquaria while clean, low organic interstitial samples can be kept in a refrigerator or at the same temperature from which the sample was originally collected. With minimal effort (e.g., air stones, refilling with tap water), such samples may provide gastrotrichs for more than a year, but are generally most species-rich just after collection. Subsamples can later be taken from the aquaria with a large pipette and transferred to a petri dish. Individual gastrotrichs can then be picked out using a dissecting microscope and a very fine micropipette and then mounted alive between a glass slide and a cover slip. Gastrotrichs are usually very active and need to be slowed down under the coverslip for proper identification. This can be accomplished by adding an anesthetic, by replacing the water with methylcellulose, by gently squeezing with a rotocompressor, or by removing some water with filter paper. For anaestheization, cocaine or MS 222 were the traditional choices, but individual specimens are now usually treated with 1–2% magnesium chloride or by bleeding 1.8% neosynephrine under the cover slip. This generally immobilizes the animal long enough to allow for proper identification and photography. To prevent squashing, small amounts of modeling clay can be applied to the edges of the cover slip. Once the animal is mounted, it needs to be oriented in a dorso-ventral fashion. Documentation of gastrotrichs is preferably done with a high-resolution DIC (differential interference contrast) microscope or a phase contrast microscope connected to a digital camera or video camera. Small amounts of water should be added to the slide as it evaporates to prevent artifacts such as body distortion. Both low- and highmagnification photographs should be used to document the external and internal anatomy. Morphometric data should be obtained with the use of an ocular micrometer. Standard

221

measurements and observations should include the following: body length; pharynx length; length of adhesive tubes; length of ciliary tufts; presence, size, and distribution of cuticular features; and reproductive status. Once the animal is sufficiently documented, it may be recovered from the slide by gently adding additional water and removing the cover slip with a fine needle; the animal can then be sucked up with a fine pipette. For archival and subsequent morphological analysis, the animal can be preserved in a 10% borax buffered formalin solution and stored at 4 °C for future studies. Whole mounts can be made by taking the specimen through a series of glycerol and alcohol to pure glycerol (Kånneby et al., 2009). By adding a small amount of formalin to the glycerol, the risk of bacterial growth and degeneration of tissue of the specimen decreases. However, for taxonomic work, photo- or videomicroscopy of living animals may provide better permanent documentations of species characters than permanently mounted specimens. Most museums now permit the deposition of digital photographic and video documentation on CDs and DVDs. For molecular studies, specimens should be fixed in 95% ethanol. Since the whole animal needs to be used for extraction of a sufficient amount of DNA, it is important to have photographs or video recordings of the actual specimen, which can then serve as a voucher. Because very few gastrotrichs can be identified with a dissecting microscope, this also prevents wrongly assigned DNA sequences in global databases. Freshwater gastrotrichs have been successfully cultured (Bennett, 1979) on 0.1% malted milk, raw egg yolk, baked lettuce infusion, and baker’s yeast. Hummon (1974) described a procedure for starting individual cultures of known-age animals from eggs that may be especially useful for bioassay work.

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SECTION | III  Protozoa to Tardigrada

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